Television receivers are increasingly making use of digital signal processing, for example for 100 Hz or 120 Hz reproduction. For this purpose, the analogue video signals are digitized using an A/D converter, digitally processed and thereafter reconverted, using a D/A converter, into analogue signals which are represented on a picture screen.
Present-day television receivers use a line-coupled clock system as clock for the A/D and D/A converters, since this produces an orthogonal pixel array for signal processing on the picture screen. The disadvantage of such a clock grid resides in its problematical generation, in particular whenever video recorders are used as signal source, because they emit unstable or non-standard signals.